If Specter truely is with us, then he will lend a perception of moderation in any heated confirmation hearings and will be an important vote in favor of any nominee to the bench. If he can not be trusted, well, then he will be the back stabber. But Santorum is not a back stabber in my opinion. He has been an excellent Senator.
The way I look at it is, if Specter is confirmed, he is Santorum's responsibility. Anything which he does wrong will be justly held against Santorum.